1. Field of the Invention
The invention relates to a worm reduction gear and a steering system.
2. Description of the Related Art
In a worm reduction gear in an electric power steering system that transmits a rotating output from an electric motor to a steering shaft, a worm shaft coupled to the electric motor so as to be driven by the electric motor is meshed with a worm wheel coupled to the steering shaft. Various structures have been proposed in which a bearing supporting an end of the worm shaft is biased toward the worm wheel by a bias member, in order to suppress possible backlash between the worm shaft and the worm wheel (see, for example, Japanese Patent Application Publication No. 2002-67992 (JP 2002-67992 A).
In JP 2002-67992 A, movement of the end of the worm shaft is guided by a cylindrical guide member press-fitted in a fitting hole in a housing and disposed around the bearing. The guide member is formed of a material such as a synthetic resin.
The press fitting causes a relatively high stress to be exerted on the guide member. Long-term use of the guide member in this state causes the guide member to be subjected to creep and plastically deformed. This may reduce a holding force that allows the guide member to be held in the fitting hole.